Y chromosome: together with the X chromosome, carries the genetic information that determines the sex of an organism. Males have an X and a Y chromosome, females two X chromosomes.
Yerkes-Dodson law: a correlation between task performance and an optimal level of arousal, whereby performance will first improve as a person's arousal level increases but then reach a point at which further arousal (possibly in the form of stress) will result in a decrease in performance.
Zeitgebers: external cues that help to maintain circadian rhythms by synchronising the activity of body’s organs and glands.
Zeitgeist: refers to ideas, trends and values that are dominant in a culture at a particular point in history.
Zone of proximal development: the distance between a child's current abilities (when working unaided) and their potential abilities or what are able to achieve under expert guidance.
Zygote: a fertilised cell that is produced after a sperm fertilises an egg.